Electrical measuring instrument.



No. 730,698. PATENTED JUNE 9, 1903.

. W. H. PRATT. ELECTRICAL MEASURING INSTRUMENT.

nnnnnnnnnnnnnnnnnn PT. 10, 1902.

witnesses: DJ William Fr'abf.

No. 730,698. PATENTED JUNE 9, 1903.

' W. H. PRATT. 7

ELECTRICAL MEASURING INSTRUMENT.

A P P L I O A T I 0 N r I L E D S E PT. 10, 1902.

i N 0 M O D E L 2 S H E E T S S H E E T 2 witnesses: Inventor WilliamHprabt,

UNITED STATES Patented June 9, 190$.

PATENT OFFIcE.

WILLIAM H. PRATT, OF LYN N, MASSACHUSETTS, ASSIGNOR TO GENERAL ELECTRICCOMPANY, A CORPORATION OF NEW YORK.

ELECTRICAL M EASURING INSTRUMENT.

SPECIFICATION forming part of Letters Patent N0. 730,698, dated June 9,1903.

Application filed September 10,1902. Serial No. 122,849. (No model.)

To all whmn it may concern:

Be it known that 1, WILLIAM H. PRATT, a citizen of the United States,residing at Lynn, county of Essex, State of Massachusetts, have inventedcertain new and useful Improvements in Electrical Measuring Instruments,of which the following is a specification.

This invention relates to electrical measuring instruments of thattypein which an electromagnetic field is employed for the movingelement. Instruments of this kind are'sometimes called springlessinstruments, from the fact that the moving element has its return orzero-seeking force determined by a small body of magnetic metal shiftedat an angle to the lines of force by the torque of the moving element.

My invention embodies an instrument of the astatic type, in which themoving element is inclosed in a composite field composed of fourpole-pieces arranged in inverse order with respect to the moving elementto guard against errors of indication due to stray fields of force. Ininstruments of this kind as heretofore constructed a plurality of coilsto magnetize the field-poles have been employed.

It is one object of my invention to reduce the amount of wire necessaryand render the instrument more compact, as well as to improve itsappearance,by providing a structure in which the several pole-piecesforming the astatic couple may be polarized by a singlemagnetizing-coil. To effect this result, I provide acore on which themagnetizing-coil is wound,mounting on one of its ends a magnetic yoke,the ends of which form one member of each astatic pair, and bifurcatethe other pole or core end at or near the yoke of the magnet to form theother poles of the astatic couple. This arrangement decreases the amountof wire necessary to polarize the" field, renders the construction verycompact, and greatly enhances the appearance of the instrument.

My improvements also embody a construction by which a differentialindication may be given and two circuits compared. To effect thisresult, I provide two sets of moving coils,

both mounted on the same arbor, but separated from each other by aninsulating medium, such as a glass or mica plate, the compound elementthus formed moving in the same magnetic field.

- The several features of novelty will be hereinafter morefullydescribed and will be definitely indicated in the claims appended tothis specification.

In the accompanying drawings, which illustrate my invention, Figure lisa plan view, with the cover removed, of an instrument embodying myimprovements. Fig. 2 is a sectional view on a magnified scale, showingthe relations of the moving element to the fieldpoles and showing alsothe mounting of the several parts. Fig. 3 is a side elevation of themagnetic core forming the compound or astatic field. Fig. at is asectional view on a plane indicated by the line 4= 401 Fig. 3 looking inthe direction indicated by the arrow, and Fig. 5 is a sectional view ona plane indicated by the line 5 50f Fig. 3 lookingin the directionindicated by the arrow.

l-will first describe the magnetic structure.

Measuring instruments as commonly used are provided with permanentmagnets. The

instrument to which my improvements relate is one with whichelectromagnets are employed and which aifords a much stronger torque andwhich for that reason is more sensitive, reliable, and accurate thaninstruments having aweaker field. Instruments of this kind also do notrequire recalibration, as do permanent-magnet instruments, in which aweakening of the magnets occurs in time. In order, however, to provide aconstant zero-seeking torque, notwithstanding possible fluctuations ofvoltage in the magnetizing-current, these instruments are provided witha magnetic return-piece in which the body of magnetic material employedis saturated at a current density greatly below the range of fluctuationof the magnetizing-current. "The return torque, therefore, is constantatany given angular deflection. Such return-pieces are indicated at 1 and2 in Fig. 2 of the drawings, being constituted in the type shown of apiece of brass or other non-magnetic metal electroplated or otherwisecoated with a thin layer of magnetic material--such, for instance, as

iron or nickel. The cross-section of the layer is so thin that it issaturated at low current density. One or more of these pieces aremounted on the arbor which carries the movable element, the latterturning in an astatic field formed of two pairs of poles NS and N S. Twoof these pole-pieces, as N N, are connected to core extensions of thecore 3 of the electromagnet, being screwed or otherwise secured toparallel extensions or arms of an iron yoke 4, mounted on the core. Theother pole-pieces S S are similarly secured to extensions 5 5 of an ironyoke 6, which is screwed to the other end of the core. The coil 7isslipped on the core and the yoke 4 then driven in place. The arbor ofthe moving element comprises a rod 8, mounted in jewel bearings in theusual manner, the bearings being supported'in a non-magnetic frame 9,carried on the field-poles. Insulated binding-posts 1O 10 11 11 aremounted on the frame and connected with suitable copper binding-posts 1212, &C., supported in the instrument-casing, a pair of terminals beingprovided for a m'agnetizingcoil and for each of the coil. systems of thecompound moving element. An insulated sleeve 13 is secured on the arbor8, on the lower end of which is carried a metal sleeve 14, whichsupports a spider 15, which is fastened in an aluminium or other metaldisk 16, carrying two coils of light wire nesting in a pressed recess inthe sides of the disk. These coils are indicated at 17 17. A similardisk is arranged just above the disk 16 and is separated therefrom by agood insulating material, such as glass or mica, as indicated at 18.Hard-rubberor other insulating rivets l0 clamp the two armature disksfirmly together. The leads from the lower armature-coils are carried tothe inner end of the flexible spiral conductors, as indicated in thedrawings, which connect with the binding-posts 1010. The moving coils ofthe upper armature connect through suitable leads with flexibleconducting-spirals 2O 21, which similarly communi cate withbinding-posts ll 11. Thus the instrument may be connected with twocircuits which are to be compared, the terminals of one circuit leadingto the bindingposts 11 ll and of the other to the bindingposts 10 10.The needle or pointer 22 is fastened to the arbor which carries themovable element and a scale-plate, provided, as

- usual, with instruments of this kind.

ment, since the magnetic return-pieces 1 2 are saturated for much weakercurrents than normally flow, and consequently when the voltage declinesor rises the moment of the zero-seeking force is changed due to themagnetic return-piece, and the change of fieldmagnet strength affectsalike both the returnpiece and the movable element. The polepieces maybe chamfered or otherwise shaped so as to produce a symmetrical fieldabout the armature. It will thus be seen that the moving element and themagnetic return piece or pieces are mounted in the same field and thatthis field is astatic and is polarized by a single core. Anyfluctuationof polarizing magnetism due to change of voltage in the magnetizing-coilaitects the same component of the torque on the moving coil and on themagnetic return-piece alike.

While I have'shown and described my invention as applied to aninstrument in which the magnetic field is produced by an electromagnet,yet it is evident that it is equally well suited to an instrument inwhich the magnetic field is due to a permanent magnet.

What I claim as new, and desire to secure by Letters Patent of theUnited States, is

1. An electrical measuringinstrument having a magnetic return-piece forits movable element, provided with a plurality of pairs of poles formingan astatic system, all polarized by a single core. 7

'2. An electrical measuring instrument having its movable element and amagnetic re-- turn-piece to provide a zero-seeking force both mountedwithin an astatic field formed by a plurality of poles polarized by asingle core, and a coil for magnetizing the core.

3. A differential electrical measuring instrument comprising twoopposing movable coil systems, a common arbor upon which they aremounted, and a common magnetic field embracing both coil systems.

t. An electrical measuringinstrumentcomprising a double armature, eachhaving its own coil system, an insulating-spacer between the twosystems, and a common magnetic field embracing the two.

5. An electrical measuringinstrumentcomprising an electromagnetic field,a common core provided with a plurality of pairs of polepieces formingsaid field, a movable element, and a magnetic return-piece mountedwithin said field.

6. An astati'c measuring instrument comprisinga field-magnet having apair of extensions for each pole of the magnet juxtaposed to form anastatic'field, and a movable element mounted in said field.

In witness whereof I have hereunto set my hand this 6th day ofSeptember, 1902.

WILLIAM H. PRATT.

Witnesses:

DUGALD McK. MoKiLLor, JOHN J. WALKER.

